1. Field of the Invention
The present invention relates to a device for detecting the signal (hereinafter called signal dectecting device) from a signal source oscillating with a certain determined frequency (for example an infrared-ray light emitting diode), particularly the compensation circuit of such a device.
2. Description of the Prior Art
In the case where the input signal of the signal detecting device (for example the active type automatic focussing device having a light source for a camera, which is so designed that a light is emitted from a light source forward of the object, whereby the light reflected from the object is made use of for focus detection) for detecting the weak signal from a signal source (for example, an infrared-ray light emitting diode) oscillating with a certain determined frequency, a synchronization detecting system is normally adopted in order to improve the measuring accuracy.
Because the signal source of the above-mentioned signal detecting device oscillates with a certain determined frequency, the output of the power source battery fluctuates in synchronization with the oscillation, whereby further, in synchronization of the fluctuation of the output voltage, the driving voltage of the signal detecting circuit also fluctuates. Consequently, when the above-mentioned system is made use of, the wrong measurement due to the fluctuation of the driving voltage can not be avoided, although the noise component in the input signal can be eliminated.
Thus, there has been an attempt to connect a filter to the power source of the signal detecting device to check the output fluctuation of the power source battery. As a result, in order to obtain a high precision measurement result, the construction of the filter becomes complicated, and the manufacturing cost becomes unduly high.